Anisotropic continuous casting mold



April 1, 1969 WQRSMITH UAL 3,435,881

lANISOTROPIC CONTINUOUS CASTING MOLD Filed Jan. '3; 1967 United StatesPatent O U.S. Cl. 164-283 4 Claims ABSTRACT F THE DISCLOSURE Thisdisclosure relates t0 a continuous casting mold made of anisotropicmaterial and having an inner liner made of vitreous graphite.

This invention relates generally to molds adapted for continuouslycasting primarily metals, or metal alloys, and particularly relates to amold ofanisotropic material having an inner lining impervious to liquidmetals.

The advantages obtainable by continuously casting metals are well knownin the art. Thus, the number of steps required in treating the metal isgreatly reduced. There is also a savings in labor, a reduction in thequantity of material which is being treated which results in a savingsof storage space as well as in a reduction of the time required toprocess the metal and an increase in the yield of metal.

On the other hand, there are still problems attendant to the use of thecontinuous casting of metals. The primary problem is the removal of heatfrom the cooling metal during steady state of operation, This means thatthe heat must be transferred from the liquid metal contained in the moldinto a suitable heat sink through the mold so that the metal solidiesrelatively rapidly to permit further handling. Obviously the morerapidly the heat can be removed from the solidifying metal, the morerapidly the lmetal can be cast. One of the limitations of the presenttechniques is the relatively slow casting rate which necessitates theprovision of several molds disposed in parallel and arranged to be fedfrom the same ladle or tundish. i

These and related problems have been solved by the anisotropic moldliner disclosed in the copending application to James W. Warren, Ir.,entitled Anisotropic Mold Liner for Continuous Casting of Metals, SerialNo. 541,159, iiled on April 8, 1966, and assigned to the assignee of thepresent application. According to the Warren application the mold ismade of or lined with a refractory material having anisotropicheat-conducting properties such, for example, as pyrolytically depositedboron nitride (BN) or mica. However, it has been found that pyrolyticgraphite is particularly adapted for this purpose, In practising theinvention disclosed and claimed in the Warren application, it has beenfound that the casting mold must be constructed of individual plates ofpyrolytic graphite stacked on topof each other.

Pyrolytic graphite is deposited from a vapor containing carbon atelevated temperatures in random layers which are disposed likedisarranged stacks of cards. This is the reason Why pyrolytic graphitehas highly anisotropic characteristics. Its mechanical, thermal andelectrical properties depend upon the direction. It has becomeconventional practise to designate as a-b axes, which in turn define aplane, those'in which the graphite is deposited. The c-axis is at rightangles to the a-b plane. Pyrolytic graphite conducts heat very well inthe a-b plane but is highly insulating in the c-axis or direction. Thus,the heat conductivity of pyrolytic graphite is about 250 times as greatin the a-b plane as in the c-direction.

Pyrolytic graphite is deposited from a vapor which may be a chemicalcompound. This may, for example, be effected by dissociating methane(Cl-I4) under the inuence of heat. This is preferably done in a vacuumfurnace at a pressure which may vary within a wide range but may, forexample, be between about 1 and 10 mm. 0f mercury. The temperature ofthe furnace may also vary within a wide range but preferably is around2200 F. The manner of depositiong pyrolytic graphite is, of course, Wellknown in the ait.

Plates of pyrolytic graphite which are made in accordance with theproces diclosed hereinabove, have their a-b planes oriented in the moldso as to remove heat rapidly from the molten metal through the walls ofthe mold. On the other hand, the c-axis is oriented so that `it servesas a thermal barrier between the container of liquid metal such as atundish or ladle and a suitable heat lsink which may, for example, be awater-cooled copper lock.

It is extremely diicult and expensive to produce pyrof lytic graphite inthicknesses greater than 3A. Plate of Ipyrolytic graphite 1/2 thick onthe other hand are standard production items, When the material isdeposited aS described above it always Orients itself so that the abplate is perpendicular to the direction of the thickness.

A continuous casting mold has a length which may, for example, be of theorder of a foot. Hence, the length of the mold is many times thethickness of a pyrolytic graphite plate which can conveniently bemanufactured. Accordingly, it is necessary to stack several graphiteplates in order to form a continuous casting mold. It is very diflicultto have a perfect match between adjacent plates, and accordingly moltenmetalmay penetrate between individual graphite plates. This metalfreezes between the plates and may cause tearing of the pyrolyticgraphite plates and of the metal surface as the metal tends to movethrough the mold.

In addition, pyrolytic graphite has a high coetlicient of thermalexpansion along the c-axis. This coeicient is of the order of 12.5 x10*6 in./in./ F. Assuming for convenience that a mold for continuouscasting were 12" long and attains a temperature of 1500" F., duringoperation it would grow in length from l2 to 12M-(12.5 X 10"*5) (12)(1500) or 12.225. Unless this rather substantial expansion is providedfor, either the mold will rupture or a gap would open in the mold andagain allow the liquid metal to penetrate into the mold. Duringexperiments which have been conducted with the anisotropic molddisclosed and claimed in the Warren application, several of thesedifficulties have been observed.

It is accordingly an object of the present invention to provide animproved mold for continuously casting metals including metal alloys.

A further object of the present invention is the provision of a thinliner of vitreous carbon between the cast metal and the anisotropic moldto prevent molten metal from penetrating into openings or gaps of themold and between plates of the anisotropic mold material. Another objectof the present invention is to provide a continuous casting mold havinganisotropic Iheatconducting properties and having a liner to allow forthe large thermal expansion of the anisotropic mold.

'In accordance with the present invention there is provided a mold forcontinuously casting metals. It should be noted that the term metal ormetals is meant to include metal alloys, such as brass, steel or thelike. This mold has a portion which is disposed substantially betweenthe area where the liquid metal is poured and the area where at leastthe outer surface of the metal is solidified. This portion of the moldconsists of a refractory material having anisotropic heat-conductingproperties. Preferably such a material consists of pyrolytic graphitealthough other material may be used instead. The refractory material isoriented so that it conducts heat relatively rapidly away from the metalacross the Wall of the mold while it conducts heat relatively slowlyalong the path of movement of the cooling metal in the mold.

Finally an inner liner of vitreous carbon is provided which covers therefractory material to provide an impervious barrier to the liquidmetal.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation, aswell as additional objects and advantages thereof, will best beunderstood from the following description when read in connection withthe accompanying drawing, wherein the single figure is a schematiccross-sectional view through a mold embodying the present invention.

Referring now to the single figure of the drawing, there is illustrateda mold generally shown at and disposed below a suitable crucible, ladleor tundish 11. The ladle 11 contains molten metal to be cast and is usedto till the mold 10 at a predetermined rate. There is further provided arefractory backing material 12 which backs the mold proper and isfollowed by a heat sink 14. The heat sink may, for example, consist of ablock of copper provided with siutable ducts l15 for passing coolingwater through the copper block in a conventional manner.

The mold is provided Iwith a liner generally indicated at 2,0 anddisposed at least between the area where the liquid metal is poured,that is, below the ladle 11 and the heat sinlk 14. 'This portion of themold 20 consists of a refractory material having anisotropicheat-conducting properties. Preferably, such a material has a ratiobetween the heat conductivity in one plane and another plane at rightangles thereto of 50 to 1 or greater. For example, pyrolytically:deposited boron nitride (BN) is a suitable material which is refractoryand has anisotropic heatconducting properties as just defined. It isalso feasible to utilize mica for this purpose. However, We prefer toutilizevfor this purpose pyrolytic graphite.

As pointed out before, the graphite consists of indi- Vidual plates ordiscs shown at 24 and which are stacked one on top of the other.

The pyrolytic graphite discs 24 are oriented in such a manner that theywill conduct heat relatively rapidly away from the liquid metal withinthe space 21 and across the wall and into the heat sink 14. At the sametime the pyrolytic graphite discs 24 will conduct heat relatively slowlyalong the path of movement of the cooling metal in the mold. In otherwords, this prevents the heat of the molten metal from the ladle 11 andabove the mold from being conducted away directly into the heat sink 14rather than permitting metal to cool slowly so that any givencross-section has a relatively uniform temperature.

To this end the a-b plane of the anisotropic material, such as pyrolyticgraphite, extends in the horizontal direction. Hence, the c-axis isdisposed along the direction shown by the arrow 22. This will accomplishprecisely what is required, namely, to prevent a rapid conduction ofheat in the vertical dtrection as shown by arrow 2-2, while at the sametime conducting heat in the horizontal direction into the heat sink 14.

It should be noted that the heat conductivity of pyrolytic graphite inthe a-b plane is equivalent to or higher than that of copper, `dependingon the temperature. On the other hand, pyrolytic graphite in thec-direction is practically an insulator of heat.

lIn accordance with the present invention there is provided a liner 26between the pyrolytic graphite discs 24 and the inner space 21 of themold. In other words, this liner 26 forms a barrier for the liquid metalso that it cannot penetrate into the spaces between individual pyrolyticgraphite discs 24 in the manner previously explained.

This liner 26 in accordance with the present invention consists ofvitreous carbon. Glassy or vitreous carbon has been described, forexample, in the British publication Nature in the issue of January 20,1962, page 261, by S. Yamada and H. Sato. Vitreous graphite is a form ofcarbon which has the characteristics of glass. lt is fragile but hasexcellent chemical properties and is impermeable to gas and, of course,impermeable to liquids such as liquid metals. Its resistance to thermalshock is very good in view of its good thermal conductivity.

Vitreous carbon is obtained by carbonization and further thermaltreatment of organic materials having strong transverse molecular bondswhich produces a coke having a large crystalline disorder andsubmicroscopic pores. This explains its low density of approximately 1.5compared to the theoretical density of graphite of approximately 2.26and the density of pyrolytic graphite of about 2.2. Vitreous carbon isnot graphitizable in the usual Sense of the term. Even upon attaining atemperature of 2500 C. or more there is little or no modification of thecrystalline structure. The material may be heat treated at a temperatureas high as 2500 C. Thus, it will easily stand the temperature of liquidcopper or steel which is usually poured at a temperature of around 2430uF. It should be noted that vitreous carbon is not wetted by liquidmetals. Hence, any molten metals cannot adhere to the liner 26.

As shown in the drawing, the vitreous carbon liner 26 preferably extendsbeyond the anisotropic mold 20 to accommodate the expansion of the mold20 with increases in temperature. The thickness of the wall of the liner26 may be of the order of 0.1". Thus, the wall thickness need not bevery thick because the liner 26 has the primary purpose to preventliquid metal from penetrating between the spaces between individualpyrolytic graphite plates 24 or between cracks in the mold. Also, theliner 26 prevents liquid metal from entering the pores of the graphiteused for more conventional continuous casting molds.

The thermal conductivity of vitreous carbon of which the liner 26consists is not as high as that of pyrolytic graphite in the a-b plane.On the other hand, it is higher than that of pyrolytic graphite alongthe c-axis. By keeping the liner 26 of vitreous carbon relatively thinthe thermal block for the transfer of heat between the molten metal andthe pyrolytic graphite may be minimized. Also the expansion of theplates 24 of pyrolytic graphite due to the high temperature of theliquid metal may readily be taken care of. Thus, the pyrolytic graphiteplates 24 may slide up or down the vitreous carbon liner or sleeve 26.

There has thus been disclosed a mold for the continuous casting ofmetals. The mold consists of anisotropic material which conducts heatrelatively rapidly away from the metal across the wall of the mold whileconducting heat relatively slowly along the path of movement of thecooling metal in the mold. This refractory material is covered by asleeve of vitreous carbon. This permits the pyrolytic graphite to expandand contract with changes in temperature without the possibility ofliquid metal entering the gaps formed thereby. Also it prevents theliquid metal from entering the gaps between adjacent plates or discs ofpyrolytic graphite. On the other hand since the sleeve of vitrous carbonmay be made relatively thin, it minimizes the impedance to the transferof heat between the liquid metal and the mold.

The invention and its attendant advantages will be understood from theforegoing description and it will be apparent that various changes maybe made in the form construction and arrangement of the parts of theinvention without departing from the spirit and scope thereof orsacrificing its material advantages the arrangementhereinbeforedescribed merely by way of example and we do not Wish to be restrictedto the specic form shown or uses mentioned except as defined in theaccompanying claims wherein various portions have been separated forclarity of reading and not for emphasis.

We claim:

1. A mold for continuously casting metals, said mold having a portiondisposed substantially between the area where the liquid metal is pouredand the area where at least the outer surface of the metal issolidified.

(a) said portion consisting of a refractory material having anisotropicheat-conducting properties,

(b) said refractory material. being oriented so that it conducts heatrelatively rapidly away from the metal across the wall of the mold andconducts heat rela tively slowly along the pathof movement of thecooling metal in said mold, and

(c) an inner liner of vitreous carbon covering said refractory materialto provide an impervious barrier to the liquid metal.

2. A mold as defined in claim 1 wherein said refractory materialconsists of pyrolytic graphite.

3. A mold as delined in claim 2 wherein said pyrolytic graphite portionhas its a-b axes oriented at right angles to said path of movement andhas its c-aXis oriented substantially parallel to said path of movement.

4. A mold as defined in claim 2 wherein said pyrolytic graphite portionconsists of a stack of individual plates.

References Cited UNITED STATES PATENTS 2,466,612 4/ 1949 Phillips et al164-283 X 3,059,295 10/ 1962 Voss Kuehler 164-283 3,076,241 2/1963Simonson et al. 164--89 X 3,210,812 10/1965 Berwick 164-282 3,304,585 2/1967 Marchlik 249-134 X 3,381,741 5/1968 Gardner 164-82 X OTHERREFERENCES Pyrolytic Graphite Engineering Handbook, 1963, GeneralElectric Company, pp. 1-5 and 24.

J. SPENCER OVERHOLSER, Primary Examiner. R. SPENCER AN NEAR, AssistantExaminer.

U.S. Cl. X.R. 249-134 gg@ UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTIGN Patent No. 3,165,881 Dated April l, 1969 Inventor-@WILLIAM H.SMITH, DANIEL M. wH-TTLEY, and EDGAR P.EATUN It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Column 2, line 13, "proces" should be proceSS-.

column une Q2, "Plate" should be P1ates--.

column 2, une 26, "plate" should be "plane".

Column 5, line '7, after "solidified", change the period to a commeSIGNED ma SEALED APR 2S 2h-P ,r'v-'. -fw're {SEAL} Attest:

Eawara M. Fletcher, 1'1" WILLIAM E. summum, m. nesting QffiwCommissioner of Patents

